The invention relates to a process for increasing the amount of oil which can be recovered from an extensively fractured oil reservoir.
An extensively fractured oil reservoir is composed of relatively small, low permeability matrix blocks separated from each other by a network of interconnected fractures (which may be supplemented by solution channels, vugs and other cavities). In such reservoirs, some oil is often found within the fractures but most of the oil is present within the low permeability matrix blocks. Although secondary recovery processes are needed to increase the oil recovery, the conventional process, such as waterflooding, gas injection or the like, are generally inapplicable in a highly fractured reservoir.
For example, a publication by S. J. Pirson, bulletin of the American Association of Petroleum geologist, Vol. 37, February 1953, page 232, discusses production problems of highly fractured reservoirs. It indicates that, in view of the tendency for the gravity segregation of fluids in the fracture network and capillary effects to trap oil within the matrix blocks, it is desirable to reduce the extent of gravity segregation by applying a high horizontal drive pressure gradient and as high a draw down (at the production well) as can be employed without undue water encroachment. Alternatively, it recommends selectively completing the wells for producing only from the lower zone (e.g., by plugging-off the upper zone) and using cyclic depressurizations followed by gradual depressurizations during production cycles.
In a publication by S. H. Raza, First Turkey Petroleum Congress, Ankara, Turkey, Dec. 14-16, 1970 proceedings, pages 27-133, November 1971, such production problems are further discussed. It mentions that, in addition to the unsuitability of waterflooding, gas injection and the like, a water-imbibition procedure is only applicable where the reservoir is strongly water-wet and then may provide only an unattractively low rate of production. If the reservoir is sealed to an extent such that fluids can be confined at relatively high pressures, a cyclic pressure pulsing process can be used.
In a cyclic pressure pulsing process one or two water pressure cycles precede at least one gas pressure cycle or a series of alternating gas and water pulsing cycles. In such processes, nitrogen, methane and carbon dioxide have been indicated to be equally effective where oil viscosity is relatively low, although the volume required for a pressurization with C0.sub.2 is significantly greater. However, such pressuring and de-pressuring steps are relatively expensive unless the total oil-free fluid filled pore space of the reservoir is small enough that it can be refilled with relatively high pressurized gas in a relatively short time.
It is known that, when injected into a subterranean reservoir and subjected to sufficient pressure, carbon dioxide becomes relatively miscible with oil. When C0.sub.2 dissolves in an oil the oil becomes a solution having a larger volume, a lower viscosity, and a lower interfacial tension against a gas. Numerous patents have proposed using C0.sub.2 as a fluid to be injected to cause a miscible fluid drive that displaces oil toward a production location. Such processes, which require a relatively uniformly permeable reservoir, are described in patents such as British Patent No. 669,216 and U.S. Pat. Nos. such as 2,623,596; 2,875,883; 2,936,030; 3,065,790; 3,120,265; 3,405,761; 3,687,198 etc.
U.S. Pat. No. 3,653,438 describes a gravity-aided miscible-drive process that is particularly applicable to a viscous oil reservoir having a high and relatively uniform permeability. An oil-soluble gas such as carbon dioxide and/or a mixture of carbon dioxide and/or a mixture of carbon dioxide and liquid petroleum gas is injected at an upper level within the reservoir while a petroleum product comprising a mixture of oil and gas is produced at a lower level. Where the oil zone overlies an active aquifer, nitrogen or any low valued gas is preferably injected into the highest point within the reservoir to maintain an overall reservoir pressure that prevents or controls the water encroachment.
However, as indicated above, such previously proposed drive or drainage processes that involve the flowing of oil through a reservoir of relatively uniform permeability are not applicable to an extensively fractured reservoir. In such a fractured reservoir the permeability is very high in the fracture network but is very low within the oil-containing rock matrix. Drive fluids flow easily through the fracture network, but bypass the oil in the matrix blocks. In addition, because of the gravity segregation of the fluid within the fractures, any undissolved gas spreads quickly to the vicinity of any production location. Therefore, if a mixture of liquid and undissolved gas is produced while an oil-soluble gas is being injected, the injected gas may be produced before any significant proportion of it has been dissolved in oil.